This invention relates generally to electromagnetic flowmeters, and in particular to a technique for stabilizing an insulating liner formed of thermoplastic material and injection-molded onto the inner surface of a metal spool having a cylindrical body provided with end flanges, the lined metal spool being incorporated into a flowmeter to define a flow conduit for the fluid being metered.
Magnetic flowmeters such as those disclosed in U.S. Pat. Nos. 3,695,104; 3,824,856; 3,783,687 and 3,965,783 are especially adapted to measure the volumetric flow rates of fluids which present difficult handling problems, such as corrosive acids, sewage and slurries. Because the instrument is free of flow obstructions, it does not tend to plug or foul. The flowmeter can be used to meter liquids without regard to heterogeneous consistency.
In a magnetic flowmeter, an electromagnetic field is generated whose lines of flux are mutually perpendicular to the longitudinal axis of the flow tube through which the fluid to be metered is conducted and to the transverse axis along which the electrodes are located at diametrically-opposed positions with respect to the tube. The operating principles are based on Faraday's law of induction, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field will be proportional to the velocity of that conductor. The metered fluid effectively constitutes a series of fluid conductors moving through the magnetic field; the more rapid the rate of flow, the greater the instantaneous value of the voltage established at the electrodes.
In order to provide a compact and readily installable electromagnetic flowmeter whose weight and dimensions are substantially smaller than existing types, the Schmoock U.S. Pat. Nos. 4,253,340 and 4,214,477 disclose a highly compact flowmeter which, in spite of its reduced volume and weight, is capable of withstanding high fluid pressures. In the Schmoock flowmeter, use is made of a non-magnetic metal spool of high strength whose inner surface is lined with insulating material to define a flow conduit for the fluid to be metered, the spool having a cylindrical body with end flanges at either end and a pair of diametrically-opposed bosses midway between the end flanges to receive the meter electrodes.
The metal spool also serves to withstand fluid pressure as well as the compressive forces to which the meter is subjected by bolts bridging the flanged ends of upstream and downstream pipes between which the unit is interposed.
Surrounding the Schmoock spool and concentric therewith is a cylindrical housing formed of ferromagnetic material. The housing is provided with annular end plates that are joined to the corresponding end flanges of the spool to define an inner chamber. Integral with the housing are two magnet cores which are placed at diametrically-opposed positions along an axis which is normal to the longitudinal axis of the housing, coils being wound on these cores. A pair of electrodes are mounted on the spool at diametrically-opposed positions along a transverse axis at right angles to the core axis. The inner chamber is filled with a potting compound to encapsulate the electromagnets and the electrodes, the housing serving as a mold for this purpose.
Insulating liners for electromagnetic flowmeters are usually molded of fluorocarbon materials such as PTFE, PFA and FEP. Because fluorocarbons are non-reactive with virtually all corrosive fluids, they have properties appropriate to liners for flowmeters. When injection-molding plastic liners into the body of metal spools of the type included in flowmeters disclosed in the Schmoock patents, certain problems are encountered, particularly when the thermoplastic material used to form the liner is of the type having reinforcing fibers therein.
In order to solve this problem, the above-identified related patent application whose entire disclosure is incorporated herein by reference, discloses an injection-molding technique in which the metal spool is supported within a mold which defines a liner cavity conforming to the inner surface of the cylindrical body and to the faces of the end flanges, the liner cavity communicating with cavities conforming to the inner surface of the electrode bosses. When the mold cavities are filled with molten thermoplastic material which then cools and solidifies the resultant liner conforms to the body and flanges of the spool and also to the inner surface of the electrode bosses.
While the injection-molding technique disclosed in the copending application tends to reduce shrinkage in the direction of the spool axis and thereby prevent the liner flanges from pulling away, an annular gap is developed between the liner and the cylindrical spool body. This gap is created because of the differential shrinkage between the liner and spool body as the thermoplastic material cools and solidifies.
As a consequence of this gap, fluid to be metered which flows through the liner may exert a pressure thereon sufficient to cause the liner to expand into the free space. Thus the internal diameter of the liner may be caused to vary as a function of the internal fluid pressure, this resulting in calibration shifts and possibly in liner rupture should the expansion of the liner exceed the physical limits of the material.